Minghui Zhang

A key challenge in scaling up rapid single flux quantum (RSFQ) circuits is managing the magnetic interference caused by their bias current distributions. This paper establishes a simulation-driven framework for optimizing RSFQ circuit bias current supply configurations using magnetic field uniformity as the primary metric. Through Ansys HFSS 3D electromagnetic simulations and subsequent data processing, we demonstrated that magnetic field uniformity—rather than absolute intensity—exhibits a stronger correlation with circuit success probability. We developed a quantitative scoring metric to systematically compare and select bias current configurations. Applying this scoring metric to an 8-bit central processing unit (CPU) series, we found that the fully functional version exhibits 6.0% lower magnetic field non-uniformity in the arithmetic logic unit (ALU) block compared to the non-functional one. The proposed method provides a viable approach to optimizing the layout of large-scale RSFQ circuit, specifically to the design of the bias current supply network.

The magnetic fields induced by the bias currents and the return currents flowing on a ground plane have a great influence on the performance of complex rapid single flux quantum (RSFQ) circuits. Conventional approach applies equal negative bias currents to the ground contacts adjacent to the bias supply contacts to alleviate this problem, but this strategy may not be optimal when the power supply network is complex. We propose a new test approach to adjusting the negative bias currents independently. By changing the current distribution and thereby altering the spatial distribution of the magnetic field across the chip, this approach can make the circuit work, or make the circuit work more stably. Experimental validation on two 8-bit RSFQ CPUs demonstrated the approach's efficacy: redistributing negative currents restored functionality in a nonoperational circuit and eliminated the intermittent failure in another. These results highlight the critical role of adjusting current distribution in overcoming magnetic interference, offering a practical solution for testing and optimizing the power supply of complex RSFQ circuits where conventional approach falls short.